Querying a catalog lets you extract information from the catalog. You can either look at the whole catalog, or you can index the catalog.
Indexing a catalog is similar to indexing a book: It speeds up the lookup of discrete terms.
When querying a catalog, you can refine the query by only looking for certain things in the catalog. This is called indexing the catalog. For example, if your catalog encompasses all the phone numbers of your company's employees, you could index the query (search) such that the dredger would only look for one phone number or a select group of phone numbers.